The possibility of using liquid phase instead of vapor phase oxidation for the preparation of benzene carboxylic acids was first indicated by the disclosure in U.S. Pat. No. 2,245,528 of the catalysis provided by transitional or variable valence metals, especially cobalt, in a liquid phase of saturated lower aliphatic acid at temperatures from 100.degree. to 320.degree.C. and pressures to maintain the liquid phase of the aliphatic acid. Such catalysis, according to said patent, was advantageously promoted by the use of a ketone such as methylethyl ketone or aldehyde such as acetaldehyde. Unfortunately such aldehyde or ketone promoted variable valence metal catalysis was useful only for converting mono-, di- or tri-methylbenzenes to their respective benzene monocarboxylic acids: benzoic, toluic and dimethyl benzoic acids. Two separate, later and somewhat parallel lower temperature (80.degree.-100.degree.C.) modifications of the aldehyde or ketone promoted cobalt catalysis in liquid phase of acetic acid did provide commercially feasible conversion of xylenes to phthalic acids, especially p-xylene to terephthalic acid but only at the expense of using rather high concentrations of cobalt and molar, with respect to p-xylene, quantities of acetaldehyde or methylethyl ketone promoter which were oxidized to acetic acid.
The disadvantages of using high concentrations of cobalt promoted with large quantities of aldehyde or ketone were overcome, and at the same time a greater choice of variable valence metal oxidation catalyst was made available and a wider choice of alkyl-substituted benzene starting materials for benzene di-, tri- and higher carboxylic acids was provided by the discovery of the unique promotional effect on said variable valence metal by bromine ion, provided per se or formed in situ with or without acidic reaction medium provided by C.sub.1 -C.sub.8 monocarboxylic acids having no hydrogens on a tertiary carbon such as benzoic acid and the saturated aliphatic monocarboxylic acids, preferably acetic acid. Such bromine-variable valent metal catalysis, first disclosed in U.S. Pat. No. 2,833,816 also provided, under liquid phase conditions over the temperature range of 120.degree.-275.degree.C., a substantially higher rate of oxidation (e.g., reaction duration of 2 hours or less for conversion of xylenes to high yields of phthalic acids) of alkyl side chains on the benzene ring to nuclear-substituted carboxylic acid groups and was not limited to such oxidative conversion of methyl side chains but rather also applied to much longer side chains. Also the unique effect of bromine ion was not limited to cobalt as variable valence metal but applied in general to polyvalent metals which have atomic weights in the range between 50 and 200. Such unique effect of bromine was most promounced when used in combination with cobalt, manganese, cerium (the metals of known highest oxidation potential) and with combinations of two or more of those polyvalent metals. Combinations of cobalt and manganese became preferred for commercial use. However, said patent also indicated that bromine had some unique effect on metals normally considered as having valencies which are non-variable. For example, use of bromine-bismuth catalysis caused selective oxidation of the isopropyl-substituent of p-cymene to the exclusion of oxidation of the methyl-substituent to yield p-toluic acid.
The bromine-polyvalent metal catalysis in acetic acid solvent has been in commercial use in many countries for the manufacture of terephthalic acid from p-xylene for more than 14 years and has progressed to the point of producing a crude terephthalic acid product of 99.5-99.6 weight percent purity in yields of 94-96 percent of theory in reaction durations of 40-60 minutes. But, in the absence of acetic acid solvent, best yield of a single phthalic acid (e.g., terephthalic acid) on a once through basis of the xylene amounted, according to U.S. Pat. No. 2,833,816 to about 20 weight (12.8 mole) percent. Most development attention to achieve such high commercial yields and quality has been given to the catalysis afforded by combinations of bromine with mixtures of cobalt and manganese as polyvalent metal components of the catalysis. However, no attention has been given to the effect on such otherwise highly effective catalysis by metals whose valencies, in general, have been considered to be non-variable in spite of earlier indications that bromine, unique of the promoters, tended to promote them from no activity to some activity as oxidation catalysts.